The invention concerns a method of producing thermally insulating glass sheets which have a good transmission behavior in the visible spectrum range and good reflection behavior toward heat radiation. The glass sheets are useful, for example, as window panes or glazing. The invention involves applying coatings (a coating system) to the glass sheets.
Window glazing of the above kind is intended to permit the transmission of the greatest possible amount of visible light and to retain, in the space bounded on the outside by the window concerned, the greatest possible amount of radiated heat. This arrangement is intended, particularly in winter, to prevent expensive thermal energy from becoming dissipated. In this connection it has to be borne in mind that an uncoated window pane acts like an opening for heat radiation. Known coating systems of this kind are therefore referred to in international parlance as "low E coatings".
De-AS No. 22 21 472 discloses a method of producing a thermally insulating glazing means by the vacuum deposition of silver and aluminium, among other materials, wherein the silver coating is intended to effect a reduction in the transmission coefficient of as much as 25% to 75% and wherein the aluminium coating intended to bring about a further reduction in the transmission coefficient of from 20% to 50%. Coatings of this kind do not have a good transmission behavior in the visible range, and the thicknesses of the silver coating and the aluminium coating are substantially greater than 50 nm. Thus, the metallic layers are pronouncedly "thick". Because use is made of vapour deposition, the aluminium layer is not altered appreciably during the production process even when an oxide layer is subsequently applied, by vapour-deposition, to the silver-aluminium coating system.
DE-OS No. 24 07 363 discloses a method of producing a semi-reflecting glazing which likewise does not exhibit good transmission behavior in the visible spectrum range. During the process of manufacture, metal coatings, including silver, are deposited directly on the sheet of glass, and an oxide coating, forming a protective layer, is then deposited by cathodic atomization on, for example, the silver coating. It has been found, however, that a sufficiently good transmission behavior cannot be achieved by a method of this kind, the expression "sufficiently good transmission behavior" being understood as meaning a maximum in the measured transmission curve of at least 80% and preferably at least 85%.
In connection with the production of heat-insulating window panes, it is known from DE-OS No. 31 30 857 to embed a layer of silver between two layers of oxide which consists of one of the materials lead oxide, antimony oxide and tellurium oxide or a mixture or alloy of these materials. All of the coatings are intended to be applied by cathodic atomization. It has been found, however, that a coating system of this kind is likewise incapable of providing the required good transmission behavior in the visible spectrum range. It was observed, for example, that with a satisfactorily good reflection behavior in the thermal radiation range, the transmission behavior deteriorated because of a corresponding absorption, and light dispersion could also be observed when the consumption of silver per unit of area was high.
The coating layer that is actually effective for good heat reflection is the silver coating layer. This should be as thin as possible and applied in such a way that its thickness is extremely uniform. Examinations under the electron microscope have shown that this requirement can be fully met by a cathodic atomization process, at least up to the stage at which the first oxide coating and the silver coating are applied. Photographs taken with the electron microscope show, at this stage, a solid silver coating of uniform thickness. The optical properties of such a two-layer system were found to be good in all cases if the effect of the last oxide or protective layer, yet to be applied, is ignored. However, as soon as this last oxide layer was applied by cathodic atomization, an abrupt deterioration of the optical properties was observed, in that the transmission behavior as well as the reflection behavior became notably poorer. Then, in order to improve the reflection behavior for the heat radiation and to return it to the required level, the quantity of silver per unit of area was increased, and this caused further deterioration in the transmission behavior.
Examinations of a coating system of this kind under the electron microscope have led to the finding that the silver coating quite obviously lost its compactness under the effect of the cathodic atomization process during the application to the last layer of oxide, and that it contracted to form isolated drops of silver. This effect was confirmed by measuring the electrical surface resistance of the coating. When the last oxide coating was applied, the surface resistance rose to a multiple of its original value. Even a considerable increase in thickness of the layer of silver did not result in the closing up of the spaces between the "silver islands", but ultimately brought about a dulling of the coating, which is quite obviously attributable to scatter of light. The reason for the break-up of the silver coating can perhaps be attributed to the reactive glow-discharge during the application of the last oxide coating.
One aspect of the present invention involves providing a method of raising the transmission in the visible spectrum range, as well as the reflection in the heat radiation range, of a transparent substrate. In particular, the invention permits an extremely uniform level of transmission and reflection, while using the lowest possible amount of silver per unit of area of the coated window pane. Expressed in other terms, the invention involves preventing, or at least greatly suppressing, the subsequent break-up of the silver coating during the application of the last oxide coating by cathodic atomization.